JaTeCS is an open source Java library focused on Automatic Text Categorization (ATC). It covers all the steps of an experimental activity, from reading the corpus to the evaluation of the experimental results. JaTeCS focuses on text as the central input, and its code is optimized for this type of data. As with many other machine learning (ML) frameworks, it provides data readers for many formats and well-known corpora, NLP tools, feature selection and weighting methods, the implementation of many ML algorithms as well as wrappers for well-known external software (e.g., libSVM, SVM_light). JaTeCS also provides the implementation of methods related to ATC that are rarely, if never, provided by other ML framework (e.g., active learning, quantification, transfer learning).